Most trailer suspensions incorporate leaf springs, mainly for the reason that the snubbing action caused between the leaves of the spring upon its deflection reduces the need for a separate shock absorber. As is well known, most trailers are required to function under a very wide range of loads, and shock absorbers suitable for such wide ranges of loads are difficult to construct. If a shock absorber is suitable for a very heavy load, it is likely to be excessively stiff for a lightly loaded trailer.
The usual leaf spring arrangement of a trailer utilises an axle extending between two ground engaging wheels, the axle being bolted to the centre of each of two leaf springs, each leaf spring being hinged at one end to the chassis of the trailer and coupled by means of a shackle at the other end. However upon a large degree of deflection taking place, the centre of the spring moves in a longitudinal direction with respect to the direction of travel. In many instances this is not of great importance, provided the two springs are deflected simultaneously by approximately the same amount. Sometimes however one spring is deflected more than the other, (particularly when the springs are unevenly loaded) and the combined effect of flattening the spring and pivoting it about its mounting at one end causes only that wheel supported by that spring to move in a longitudinal direction (usually rearwardly) with respect to the trailer. This in turn causes the axle to be deflected from its transverse position with respect to direction of travel, and the result is that the trailer is caused to move in a direction which deviates from its previous straight line movement. This is one of the reasons why trailers tend to cause the towing vehicle to "snake" or sway while being towed.
One of the objects of this invention is to provide a trailer suspension wherein the tracking of the wheels is less likely to be influenced by uneven spring deflection.
It is already known that a suspension for a vehicle can incorporate a Y ("wishbone") shaped wheel frame wherein a coil spring is compressed by the load of the vehicle, the coil spring extending between a hinged Y ("wishbone") frame and the vehicle chassis. However these hinged frames are of the pantograph type, incorporating two pairs of bearings to control the wheel movement, and the design is such that the interface between wheel tyre and road is not displaced upon hinge deflection, but the wheel tilts transversely. The use of two pairs of bearings and the further use of a shock absorber subject to the disabilities described above, not only greatly adds to the cost of construction but also results in a construction which is not sufficiently "stiff" to avoid deviation of direction under rough road conditions.
Various alternatives have been proposed, which have met with some success.
For example, in the U.S. Pat. No. 2,062,874 issued to Filton, there was described a device wherein a vertical bearing member constrained wheel axle movement against longitudinal displacement, and in the U.S. Pat. No. 2,085,662 issued to Johnson there was disclosed an arrangement wherein some longitudinal displacement occurred upon spring deflection, but independent springing of two transversely aligned wheels avoided transverse deflection such that direction of travel was influenced.